The invention relates to a method and dual reactor system for hydrotreating a wide cut cat naphtha stream comprising heavy cat naphtha (HCN) and intermediate cat naphtha (ICN). Accordingly, a HCN fraction is hydrotreated under non-selective hydrotreating conditions and an ICN fraction is hydrotreated under selective hydrotreating conditions. The hydrotreated HCN and ICN effluents may be conducted to heat exchangers to pre-heat the ICN feed, obviating the need for a furnace.
The need for low-emissions, high-octane fuels has led to a need for fuels processes that diminish the concentration of sulfur-containing species in the fuel without substantially changing the fuel""s octane number.
Conventional fuel processes for sulfur removal include contacting a naphtha with a catalyst in the presence of hydrogen under catalytic conversion conditions. One such technique, called catalytic hydrodesulfurization (HDS), involves reacting hydrogen with the sulfur compounds in the presence of a catalyst. HDS is one process within a class of processes called hydrotreating, or hydroprocessing, involving the introduction and reaction of hydrogen with various hydrocarbonaceous compounds. Hydrotreatment has been used to remove sulfur, nitrogen, and other materials such as metals.
Cracked naphtha obtained as a product of, for example, fluid catalytic cracking, steam cracking, thermal cracking, or coking may contain a significant concentration of sulfur up to as much as 13,000 ppm. Although the cracked naphtha streams constitute approximately half of the total gasoline pool, cracked naphtha contributes a substantially higher percentage of undesired sulfur to the gasoline pool. The remainder of the pool typically contains much lower quantities of sulfur.
Hydroprocessing cracked naphtha typically results in a product having a diminished concentration of olefinic species and non-hydrocarbyl species such as sulfur-containing species, and an augmented concentration of saturated species. Relatively severe hydroprocessing conditions are generally required to substantially remove sulfur-containing species, and such severe hydroprocessing conditions are known to result in a substantial octane number reduction in the hydroprocessed product.
Some conventional sulfur removal processes attempt to overcome the octane number reduction problem by making use of the non-uniform distribution of olefins and sulfur-containing species across the naphtha boiling range. In a typical naphtha, olefins are most concentrated and the sulfur concentration is relatively low in the fraction boiling between about 90xc2x0 F. and 150xc2x0 F., i.e., the light cat naphtha or xe2x80x9cLCNxe2x80x9d fraction. Sulfur species are most concentrated and the olefin concentration is relatively low in the heavy cat naphtha or xe2x80x9cHCNxe2x80x9d boiling range, typically about 350xc2x0 F. to about 430xc2x0 F. Intermediate cat naphtha (xe2x80x9cICNxe2x80x9d) typically boils in the range of about 150xc2x0 F. to about 350xc2x0 F. and may contain significant amounts of both sulfur species and olefins. Sulfur species in the LCN fraction may be removed by caustic extraction without undesirable olefin saturation, while the ICN and HCN fractions generally require hydrotreating to remove the sulfur.
In one conventional process, the ICN fraction is hydrotreated under relatively mild conditions in order to lessen the amount of olefin saturation, while the HCN fraction is hydrotreated under more severe conditions. One disadvantage of this approach relates to the complexity and costs associated with operating two independent hydrotreating units and their associated feed pre-heating equipment.
There remains a need, therefore, for new processes for forming naphtha having a diminished concentration of sulfur-containing species, while maintaining a sufficient olefin concentration to provide a relatively high octane number.
In an embodiment, the invention relates to a method for hydrotreating heavy cat naphtha and intermediate cat naphtha streams. The method comprises hydrotreating a heavy cat naphtha feedstream having a HCN initial sulfur content and a HCN initial olefin content under HCN hydrotreating conditions effective to produce an HCN effluent at an elevated temperature having a HCN effluent sulfur content and a HCN effluent olefin content. An intermediate cat naphtha stream at an initial temperature is heated with the HCN effluent, via a heat exchanger for example, and thereby heated from the initial temperature to an increased temperature. The increased temperature ICN stream is hydrotreated under ICN hydrotreating conditions which are less severe than the HCN hydrotreating conditions to produce an ICN effluent having an ICN effluent sulfur content and an ICN effluent olefin content. In a preferred embodiment, the HCN effluent and the ICN effluent are combined, and the combined stream may be subjected to product separation procedures or conducted away from the process for storage or further processing. It is also preferred that HCN hydrotreating conditions be controlled to provide an HCN effluent having a temperature at least about 25xc2x0 F. higher than the ICN hydrotreater""s inlet temperature. More preferably, HCN and ICN hydrotreating conditions are controlled so that the HCN and ICN are both in the vapor phase (i.e., always above the dew point) during the hydrotreating operation.